Regulation of actin dynamics at filament ends determines the assembly and organization of actin cytoskeletal structures and is crucial for tissue development and function. Our research focuses on tropomodulins (Tmods), proteins that cap actin filament pointed ends, inhibiting monomer association and dissociation, stabilizing filaments and regulating their lengths. There are four 40 kDa Tmod isoforms in vertebrates, Tmods 1-4. Unique amongst all actin capping proteins, Tmods also bind tropomyosin (TM) to tightly cap TM-coated actin filaments with affinities over one thousand-fold greater than for pure actin filaments. Tmods are distinguished by different binding affinities for the numerous TM isoforms, and by different actin monomer-polymer regulatory properties. A hallmark of Tmods is their association with specialized actin cytoskeletal structures in post-mitotic, highly differentiated cells. Notably, Tmods cap actin filaments in the membrane skeleton, an extended planar filament network underlying the plasma membrane, organizing sub-domains of membrane proteins and influencing cell shapes and membrane mechanical stability. Tmods also cap the long actin filaments (thin filaments) in striated muscle sarcomeres, controlling thin filament lengths and stability, thereby optimizing contractile force in striated muscles.